Depletion of Ca2+ from the sarcoplasmic reticulum of cardiac muscle prompts phosphorylation of phospholamban to stimulate store refilling.
Nonmuscle cells have almost ubiquitously evolved a mechanism to detect and prevent Ca2+ store depletion-store operated calcium entry. No such mechanism has, as yet, been reported in cardiac myocytes. However, it is conceivable that such a mechanism may play an important role in cardiac Ca2+ homeostasis to ensure the availability of sufficient stored Ca2+ to maintain normal excitation contraction coupling. We present data that confirms the presence of a mechanism that is able to monitor the Ca2+ load of the SR and initiate a signaling process to accelerate Ca2+ uptake by the SR when store depletion is detected. Depletion of SR Ca2+ activates a protein kinase, the principal SR substrate of which is phospholamban. Phosphorylation of this SR protein promotes Ca2+ pump activity and therefore store refilling. Furthermore, a protein kinase activity associated with the SR that is inhibited by Ca2+ ions has been identified. We have measured lumenal [Ca2+] by using a fluorescent Ca2+ indicator and found that by initiating Ca2+ uptake and increasing Ca2+ load, we can inhibit the protein kinase activity associated with the SR. This confirms that a protein kinase, that is regulated by lumenal [Ca2+], has been identified and represents part of a previously unidentified signalling cascade. This local feedback mechanism would allow the myocyte to detect and prevent SR Ca2+ load depletion.
Bhogal
M S
MS
School of Biochemistry and Molecular Biology, University of Leeds, Leeds, LS2 9JT United Kingdom.
Colyer
J
J
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002451:Cell Compartmentation; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004285:Dogs; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D009206:Myocardium / Q000378:metabolism*; D010766:Phosphorylation; D010768:Phosphoserine / Q000378:metabolism; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D000099211:Phospholamban
D002118:Calcium; D002135:Calcium-Binding Proteins; D000252:Calcium-Transporting ATPases; D017868:Cyclic AMP-Dependent Protein Kinases; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D010768:Phosphoserine; D000099211:Phospholamban